The present invention relates to an evaluation method for evaluating the presence/absence of a damage to an insulating film, an evaluation device therefor, and a method for manufacturing the evaluation device.
In recent years, along with the demand for increasing the speed and performance of LSIs, the thickness of a gate insulating film of a MIS transistor has been reduced. Accordingly, how to suppress the damage to the gate insulating film due to the dry etching step of forming a gate electrode by patterning has been an important problem in the art.
A conventional method for forming a gate electrode made of polycrystalline silicon will now be outlined with reference to FIG. 11A and FIG. 11B.
First, as illustrated in FIG. 11A, a gate oxide film 102 having a thickness of about 2 nm is formed on the principal surface of a semiconductor substrate 101 made of silicon, including a p-type well 101a formed in an upper portion thereof, and then a gate forming film 103A made of polycrystalline silicon is deposited thereon. Then, a resist pattern 110 having a gate pattern is formed on the gate forming film 103A by using a photolithography method.
Then, the gate forming film 103A is dry-etched while using the resist pattern 110 as a mask so as to form a gate electrode 103B having a gate length of about 0.1 μm from the gate forming film 103A, thereby obtaining a structure as illustrated in FIG. 11B. The dry etching process is performed by using, for example, an inductively-coupled plasma (ICP) etcher.
Exemplary etching conditions are as follows:
(1) Flow amount of etching gas:hydrogen bromide (HBr):chlorine(Cl2):oxygen (O2) = 70:30:3(2) Pressure: 1 Pa(3) ICP power: 300 W(4) Bias power: 100 W(5) Etch selectivity of gate formingEtch rate for polycrystalline siliconfilm with respect to gate oxide film:film/Etch rate for gate oxidefilm = 50
Although the gate forming film 103A is overetched by about 30% in the gate pattern dry etching step, it is possible to reliably stop the etching at the gate oxide film 102 because of the sufficiently high etch selectivity. Thus, the semiconductor substrate 101 is not etched.
Along with the reduction in the thickness of the gate oxide film 102, the etch selectivity that is required for the gate forming film 103A is increasing. Nevertheless, with the advancements in the etching technology, it is now possible to obtain an etch selectivity of 100 or more.
However, the conventional method for forming a gate electrode has a problem in that a punch-through occurs in the gate oxide film 102 when the thickness of the gate oxide film 102 is reduced to be 10 nm or less. A punch-through refers to a ruptured hole 104 that occurs in a portion of the gate oxide film 102 beside the gate electrode 103B even though the etching should theoretically be stopped by the gate oxide film 102 in view of the sufficient etch selectivity. FIG. 12 illustrates the ruptured hole (punch-through) 104 occurring in the gate oxide film 102.
The cause of a punch-through will now be described with reference to FIG. 13.
As illustrated in FIG. 13, normally, when dry-etching the gate electrode 103B, an etchant (etching species) that contains halogen ionc and oxygen ionc and a depositing radicalc that contain a halogenide of silicon, which are supplied from a plasma, competitively interact with each other, thereby causing the etching to proceed while realizing a necessary etch selectivity.
Specifically, the etching of the gate oxide film 102 proceeds in a state where the flux of the etchant is larger than the flux of the depositing radical, whereas the etching of the gate oxide film 102 is suppressed in a state where the flux of the etchant is smaller than the flux of the depositing radical. As a result, in a state where the flux of the etchant is smaller than the flux of the depositing radical, the etch selectivity of the gate forming film 103A with respect to the gate oxide film 102 is high.
In this process, the depositing radical is supplied from a plasma in an isotropic manner. Therefore, in a region of the gate oxide film 102 beside the gate electrode 103B, e.g., a region within a distance of d from the side surface of the gate electrode 103B, a sufficient amount of depositing radical is not supplied due to a so-called “shadowing effect” with the gate electrode 103B of a certain height being a standing wall. In addition, in the region within the distance of d from the side surface of the gate electrode 103B, an etchant having been reflected by the side wall surface of the gate electrode 103B locally impinges on the gate oxide film 102.
Thus, even though the semiconductor substrate 101 as a whole is in a state where the etching of the gate oxide film 102 does not proceed with the flux of the etchant being smaller than the flux of the depositing radical, the region in the vicinity of the side wall of the gate electrode 103B is locally in a state where the flux of the etchant is larger than the flux of the depositing radical. Therefore, the punch-through 104 occurs in the region of the gate oxide film 102 that is in the vicinity of the side wall of the gate electrode 103B as illustrated in FIG. 12.
Incidentally, the evaluation of the presence/absence of the punch-through 104 occurring in the gate oxide film 102 is typically performed by observing the surface condition of the gate oxide film 102 by using an optical microscope or a scanning electron microscope.
However, the conventional method for evaluating a gate oxide film has a problem in that it is not possible to evaluate and observe the entire surface of the semiconductor substrate 101, and another problem in that visual observation is instable and unreliable.